1. Field of the Invention
The present invention relates to a variable magnification optical system, and more specifically relates to a compact, high-magnification zoom viewfinder optical system of the real image type which is provided separately from the photographic lens of a lens-shutter type camera or the like.
2. Description of the Related Art
In recent years, the demand has grown for cameras having greater functionality. Demand has also increased for viewfinder units, having correspondingly high variable magnification, for use in cameras which are provided with a viewfinder as a completely separate unit from the photographic system. However, in the case of zoom viewfinder systems, it becomes difficult to assure that the viewfinder will have suitable aberration characteristics over the entire zoom range in conjunction with the high variable magnification.
Examples of conventional systems utilizing a variable magnification ratio in the range of about 3 to about 4 are disclosed in Japanese Laid-Open Patent Application Nos. 2-173713, 2-173714, 2-191908, and 6-102453. A characteristic of these viewfinder systems is the provision of a positive/negative/positive objective optical system.
The conventional positive/negative/positive construction readily assures suitable aberration characteristics over the entire zoom range of a magnification ratio less than about 4, and is particularly suitable for high variable magnification in the range of about 3 to about 4. However, at a high variable magnification ratio in the range of about 5 to about 6, it is rather difficult to assure adequate aberration characteristics over the entire zoom range.
In cameras provided with a viewfinder as a separate unit completely independent of the photographic system, the viewfinder unit must satisfy the reciprocal conditions of compactness and high variable magnification. Generally, when the aspect of compactness in the overall length direction is satisfied, the aspect of high variable magnification is not satisfied; and when the aspect of high variable magnification is satisfied, the aspect of compactness is not satisfied. The length of the viewfinder objective system is important in reducing the thickness of the system in the overall length direction.
In the aforesaid conventional constructions having a variable magnification in the range of about 3 to about 4 and being provided with a positive/negative/positive objective optical system, there is a loss of compactness due to an increase in the overall length of the objective system when obtaining high variable magnification.